A typical communications satellite employing multiple spot beams requires a finite number of reflectors for the downlink, or transmit, frequencies and another set of reflectors for the uplink, or receive, frequencies. The two sets of reflectors usually contain about three or four reflectors and the reflectors are sized according to the frequencies. On board the satellite, the antenna farm typically consists of four offset reflector antennas for the downlink being located on one side of the spacecraft. The uplink reflectors, usually about two-thirds the size of the downlink reflectors, are located on the opposite side of the spacecraft.
Each set of reflectors employs dedicated feeds optimized over a narrow band. Each of the beams is produced by a dedicated feed horn. These payloads require a significant amount of real estate on the satellite. The east and west sides of the spacecraft are dedicated to the uplink and downlink spot beam payloads. This leaves only the nadir face of the spacecraft for other payloads. In addition to the number of feeds necessary for each set of reflectors, the large number of reflectors requires associated deployment mechanisms and support structures.
Attempts have been made to mitigate these problems. One approach is to use a single reflector for each frequency band and employ a large number of feed horns with a low-level beamforming network dedicated to each reflector. Each beam is generated by an overlapping cluster of horns (typically seven). This requires an element sharing network and a beamforming network to form multiple overlapping beams. However, any advantage gained by having fewer reflectors is overridden by the need for more feeds. This approach requires approximately thirty percent more feeds than the number of feed required for the conventional approach described above. Further, a large number of amplifiers and complex and heavy beamforming networks introduce additional cost and increased complexity to the system.
Another approach uses a solid reflector with a frequency selective surface (FSS) subreflector with separate feed arrays. The FSS subreflector transmits the downlink frequencies and reflects the uplink frequencies. In this approach, the number of main reflectors is reduced by a factor of two, but there is a need for complex FSS subreflectors, which require more volume to package on the spacecraft. In addition there is an increased loss with the FSS subreflector, which negatively impacts the overall electrical performance.
Yet another approach uses a FSS main reflector and dual-band feed horns. This approach employs one set of reflectors, where each reflector has a central solid region that is reflective to both frequency bands and an outer ring FSS region that is reflective at downlink frequencies and non-reflective at uplink frequencies. The electrical sizes of the reflector are different at the two bands and can be adjusted to achieve some beam coverage on the ground. This design approach is complex and very expensive. In addition there is still the disadvantage of losses associated with the FSS reflector.
There is a need for a reflector system that does not take up valuable real estate on board the spacecraft and at the same time is less complex and expensive than known methods.